1. Field of the Invention
This invention relates to an etch-bleaching process to be applied to the reversal developing of a line and half-tone image (often referred to as a dot image) and the production of a planographic printing plate.
2. Description of the Prior Art
The term "etch-bleaching process" as is used in this disclosure means a process for etching and removing portions of a hydrophilic layer having therein/on a silver image in areas where the silver image exists in/on the hydrophilic portions and the term "etch-bleaching liquid " means a treating solution for selectively attacking and rendering removable portions of a hydrophilic layer containing therein/on a silver image.
The terminology "in/on" the hydrophilic layer is used in the present disclosure to fully describe the generally encountered state of a silver image formed in a hydrophilic layer, that is, while the silver image is generally considered "in" the hydrophilic layer contain, surface-exposed areas do exist where the image may be considered "on" the hyrophilic layer.
Generally, a planographic printing plate is composed of an oleophilic portion forming image areas and a hydrophilic portion forming nonimage areas.
One known process for producing planographic printing plates is characterized by converting the image areas on the hydrophilic surface of the base plate solely into an oleophilic surface. Another process is characterized by first applying a hydrophilic layer on the oleophilic surface and then removing the part of the hydrophilic layer on the image areas to expose the oleophilic layer.
For example, British Pat. No. 1,129,366 discloses a process for producing a planographic printing plate comprising forming a silver image on a hydrophilic layer (which is provided on a base plate having an oleophilic surface) which contains a nuclei substance for the precipitation of silver in a diffusion transfer process and then removing the hydrophilic layer which is not covered by the silver layer from the oleophilic surface to expose the oleophilic base plate.
U.S. Pat. No. 3,385,701 discloses another process for producing planographic printing plates comprising bringing an exposed negative type photosensitive film in contact with a hydrophilic layer which is provided on a base plate having an oleophilic surface and which contains a nuclei substance for the precipitation of silver in a diffusion transfer process to thereby carry out a diffusion transfer developing and to form a silver image on the hydrophilic layer, and then removing the hydrophilic layer which is not covered by the silver layer from the oleophilic surface to expose the oleophilic base surface.
German Patent Publication (OLS) No. 2,048,594 discloses another process for producing a planographic printing plate comprising exposing an original on a photosensitive silver halide emulsion layer which is provided on a sheet of cellulose, organic acid ester having a hydrophilic surface and contains a nuclei substance for diffusion transfer, forming a reverse silver image corresponding to the original on the hydrophilic surface, treating the plate with an aqueous solution of hydrogen peroxide to remove the hydrophilic surface part not covered by the silver layer, thereby exposing the oleophilic surface of the ester sheet.
According to these three processes, the portions not covered by the silver layer are removed by an etch-bleaching process.
Such conventional etch-bleaching processes for treating the plate where only an etch-bleaching liquid is used require strong rubbing of the hydrophilic parts not covered by the silver layer with a sponge or absorbent cotton, resulting in the poor process efficiency and often causing scratches.
In addition, the hydrophilic parts not covered with the silver layer are not always removed and accordingly a clear print is hard to obtain.